1. Field of the Invention
This invention relates to gas analyzers of the type in which the impedance of an active element varies in the presence of the subject gas and creates an imbalance in a Wheatstone bridge measuring circuit.
2. Prior Art
Gas analyzers of the Wheatstone bridge type are well known. Some utilize a simple thermal detector while others, now more common, employ an electrically heated catalytic resistance element which actually initiates combustion of the gas. In either case the presence of the gas changes the impedance of the active element which creates an imbalance in a Wheatstone bridge measuring circuit, the magnitude of which is an indication of the percentage of the subject gas present. An example of a typical prior art analyzer is disclosed in U.S. Pat. No. 3,586,486 which has been assigned to the assignee of this invention. In analyzers of this general type, a noncatalytic compensating resistance element is placed in an adjacent leg of the Wheatstone bridge to counteract the effect of ambient conditions on the active element. While this patent describes a construction in which a constant flow of a predetermined mixture of the gas to be analyzed and a dilutant gas is drawn across the resistive elements, other analyzers expose the resistive elements directly to the atmosphere to be tested through porous flame suppressors.
Prior art analyzers employ either electro-mechanical meters or, more recently, digital readouts for displaying the gas concentration. Mechanical type movements have always been susceptible to breakage which is a serious handicap for use in rugged environments, such as mines. Mechanical meters are also subject to operator interpretation which can lead to errors in reading especially under poor lighting conditions unless adequate internal lighting is provided which increases the current draw on the batteries of a portable analyzer. Furthermore, some mechanical displays which appear to provide large full-scale indications are difficult to read in the lower more commonly used ranges and are therefore subject to even more interpretation.
On the other hand, digital type displays have failure modes which can be dangerous for applications such as the detection of methane gas in coal mines where a concentration of 2 percent or more is unsafe. For instance, failure of one segment in a digital reading of "1.8" percent can be displayed as "1.0" percent which would give the false impression that the concentration of methane was at a safe level when in fact it was approaching a critical level. In addition, digital displays draw close to maximum current while indicating zero percent by displaying "0.0" percent which requires the energization of 12 segments on the conventional digital display. Since the absence of methane gas is a common and strived for occurrence in coal mines, displays which draw maximum current at a zero reading are not as desirable as one which would draw minimum current in the most common operating range. Furthermore, some digital displays are confusing in that they can display negative numbers such as -"1.8" percent.
Some electro-mechanical meters and digital displays have a tendency to show annoying fluctuation. They may also display erratic false readings during the warm-up of the resistance elements. In addition, many of the prior art analyzers require the operation of separate switches to test the battery and to take readings.
Light emitting diodes (LEDs) are now widely used in digital displays. There has also been a suggestion that LEDs could be arranged to provide an analog display of the speed of a vehicle.